One of the important unknowns in global warming science is how the Antarctic ice sheet, which covers a land area as large as the continental U.S. and Mexico combined, would respond to a much warmer world. This ice cap, with an average thickness of 2,500 meters, would raise the sea level by 74 meters if it were to melt completely.
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Antarctica drew notice this year when National Oceanic & Atmospheric Administration satellite images captured part of the pack ice disintegrating on the Antarctic Peninsula (which extends outward toward the southern tip of South America). Between Jan. 14 and Feb. 27, an ice shelf that formerly blocked the Prince Gustav Channel between James Ross Island and the peninsula broke up.
During the same period, the northern section of the Larsen Ice Shelf also disintegrated, leaving a plume of debris extending 200 km east. In addition, a Rhode Island-sized chunk of ice broke off the Larsen Ice Shelf farther south. This iceberg, although large, was a fairly small component of what has happened overall recently in Antarctica, says David G. Vaughan, glaciologist at the British Antarctic Survey, Cambridge, England. Other ice shelves around the Antarctic Peninsula are also in retreat.
Over the whole of Antarctica, temperatures have increased an average of about 1 °C in 50 years, Vaughan says. But on the Antarctic Peninsula, temperatures have risen an average 2.5 °C since record keeping began there in the 1950s. This change could be a local manifestation of global warming, Vaughan says, or "it's possible it has nothing to do with global warming." In any event, these warmer temperatures have probably caused the disintegration of the ice shelves around the peninsula, he explains.
Scientists would also like to know whether the entire Antarctic ice sheet melted during the Pliocene Epoch, 3 million to 4 million years ago, when global temperatures were higher than they are today. Determining the degree of melting that occurred then would give some indication about how much of the Antarctic ice sheet would melt if global warming proceeds unabated, because without curbs on greenhouse gas emissions global warming is expected to eventually produce a Pliocene-like climate.
However, scientists don't even know whether the Antarctic ice sheet is
getting thicker or thinner overall, Vaughan says. "We're completely in the
dark. We really have no idea what goes into the ice sheet or what comes
out." It is not known how much snow falls, what remains, what sublimes, what
is blown away, or how much ice is lost through calving of icebergs from the
ice shelves. It is known that a lot of ice is lost from the undersides of
the ice shelves (which extend over seawater), but that is not being well
measured yet, nor are the smaller icebergs leaving the ice shelves being
counted in a comprehensive way. However, satellite altimetry data now being
collected should indicate whether the ice sheet is getting thicker or
thinner, Vaughan says.
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